wl12xx: remove sdio_test module

This module has been causing more trouble than being useful.  It only
tests the SDIO speed by connecting to the wl12xx chip and does some
throughput calculations.  It is an ugly quick hack and, if we really
want to have it as part of wl12xx we need to clean it up and implement
it properly.

A tarball of the code has been created and posted here, with some
instructions:

http://wireless.kernel.org/en/users/Drivers/wl12xx#SDIO_performance_test_module



Signed-off-by: Luciano Coelho <coelho@ti.com>